This proposal will investigate the mechanisms directing hematopoietic stem cell (HSC) fate decisions. As HSC must meet the demands for a life-long supply of all the mature blood cells, fate decisions have to be tightly regulated. As HSC differentiate to increasingly mature cells, self-renewal capability is progressively lost. Of particular interest is how HSC interactions with the bone marrow microenvironment affect the balance of self-renewal and differentiation. Our cDNA array analysis of the three most immature murine hematopoietic subpopulations revealed that many components of cell communication complexes are differentially regulated. Here, the functional role of particularly interesting molecules will be tested. In Aim 1 we will assess the use of the adhesion protein ESAM1 as a specific HSC cell surface marker, and determine the function of ESAM1 in HSC biology using existing ESAM1 knockout mice. Aim 2 will test whether newly discovered receptor-ligand pairs play a role in HSC migration as they do in non-hematopoietic systems. Together, these aims will provide novel insights into the regulation of HSC location, migration and cell fate decisions by both microenvironmental and HSC intrinsic mechanisms. These studies are relevant to bone marrow transplantation, autoimmune disease and leukemia. [unreadable] [unreadable] [unreadable]